Heretofore, cold-rolled steel sheets have been used for home electrical appliances and automobile outer panels. However, recently, Al—Mg alloy sheets with high strength and excellent formability have been proposed from the demands for reduction of weight.
For example, Japanese Patent Publication (A) No. 07-278716 proposes to define the Mg content as 2.0 to 6.0 mass %, limit the Si content and Fe content to 1.5 mass % or less, continuously cast the melt to a slab thickness of 1 to 10 mm, and make a cooling rate 10° C./sec or more so as to make intermetallic compounds finely disperse in the matrix to obtain aluminum alloy sheet for forming with superior mechanical properties.
However, the aforementioned document describes evaluation of the average precipitate size, mechanical properties, and formability, but no description is seen concerning recrystallized particle size and surface roughening. In addition, the total reduction by cold rolling is only limited to being preferably 50% or more in order to finely disperse the intermetallic compounds. The other manufacturing processes are not particularly limited.
In this way, the technology in casting Al—Mg alloys with a thin slab by twin roll casting so as to finely disperse the intermetallic compounds in the matrix to produce an aluminum alloy sheet for forming with superior mechanical properties has been known in the past.
However, to further make the formability higher, it was necessary to further make the size of the intermetallic compounds smaller and to improve the surface roughening of the sheet surface after forming.